Oil and Gas Fracture Liquid Tracing with Oligonucleotides

ABSTRACT

Methods of tracing fracking liquid in oil or gas bearing formations using plural unique oligonucleotide markers. Method includes pumping the plural volumes of fracking liquid, each marked with a unique oligonucleotide, into the formation, thereby defining plural fracture zones in the formation, and, pumping fluids out of the formation while taking plural fluid samples. Then, analyzing the concentration of the unique oligonucleotides in each of the plural fluid samples, and, calculating the ratio of each of the plural volumes of fracking liquid recovered for each of the plural fluid samples according to the concentration of the unique oligonucleotides present in each of the plural samples. And, then, establishing the quantity of the plural volumes of fracking liquids removed from the plural fracture zones.

RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 13/956,864 filed on Aug. 1, 2013.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to hydraulic fracturing of geologicformations in hydrocarbon wells. More particularly, the presentinvention relates to tracing the movement and recovery of hydraulicfracturing liquids pumped into oil and gas wells using plural uniqueoligonucleotides tracing compounds, which correspond with pluralfracture stages and zones within a geologic formation.

2. Description of the Related Art

Oil and gas are removed from geologic formations by drilling a well borefrom the surface. A well casing is inserted into the well bore, which isthen perforated so that oil and gas can flow from the adjacent geologicformation into the well casing. The oil and gas may flow upwardly undernatural pressure in the formation, but more commonly they are removedusing an artificial lift system, such as the well known sucker-rod pumpand surface-mounted pump-jack arrangement. In order to maintainproduction over an extended period of time, there must be sufficientformation porosity and pressure so that the oil and gas naturally flowfrom the hydrocarbon bearing geologic formation, through the casingperforations, and into the well casing.

As exploration has expanded into regions where there is insufficientporosity in the oil and gas bearing formations to sustain production,engineers have developed hydraulic fracturing techniques that produceartificial porosity, through which the formation oil and gas can flowinto the well casing. Hydraulic fracturing is the fracturing of rockstructures adjacent to the well casing perforations using a pressurizedliquid pumped down the well casing from the surface. Hydraulicfracturing, or hydrofracturing, also commonly referred to as “fracking”,is a technique in which fresh water is mixed with sand and certainchemicals, and then the mixture is injected at high pressure into a wellcasing to create small fractures in the formation. This liquid mixtureis referred to as fracking liquid. These small fractures enableformation fluids, such as gas, crude oil, and brine water to flow intothe well casing. Once the fracking process is completed, hydraulicpressure is removed from the well. The formation rock naturally settlesback to its original position, but the small grains of sand, referred toas proppants, hold these fractures open so as to yield the desiredartificial porosity. Fracking techniques are commonly used in wells forshale gas, tight gas, tight oil, coal seam gas, and hard rock wells. Thefracking process is only utilized at the time the well is drilled andplaced into production, but it greatly enhances fluid removal and wellproductivity over the life of the well.

The sequence of events implemented to place a typical oil or gas wellinto production generally consists of, drilling the well bore,installing the well casing, perforating the casing, hydrofracturing thehydrocarbon bearing formation, installing an artificial lift system,recovering the hydraulic fracturing liquid, and then producing oil andgas from the well. It is significant to note that the presence of thefracturing liquid in the formation interferes with oil and gasproduction, and that removal of the fracturing liquid is a technicalchallenge for operators, and one that must be accomplished promptly, andto a reasonable degree of completion before oil or gas production fromthe well can commence. This disclosure is primarily concerned with thehydraulic fracturing process and the removal, or other disposition, ofthe hydraulic fracturing liquid (also referred to herein as “frackingliquid”). The types of wells contemplated herein include common verticalwells and wells in which horizontal drilling is used to traverse ageologic formation so as to increase productivity. In fact, hydraulicfracturing is now commonly employed in wells having horizontal boresthrough gas producing formations. An example of this is the BarnettShale formation in north Texas, a region that covers approximatelyseventeen counties and contains natural gas reserves proven to include2.5 trillion cubic feet, and perhaps as much a 30 trillion cubic feet ofrecoverable reserves.

The effectiveness of the hydraulic fracturing process, as well as theflow and disposition of the fracking liquid, is of critical importanceto the well operator. Since the fracking process occurs far below thesurface and is therefore difficult to monitor, any data that confirmsthe extent of the fractures or indicates the flow and movement of thefracking liquid is helpful in the operation of that well, and is alsoinformative with regard to similar wells that may be drilled in the sameoil field. A technique used to determine the flow and movement of thehydraulic fracturing fluid is called tracing. The tracing processinvolves placing a marking additive (hereinafter a “tracer”) in thehydraulic fracturing liquid before it is pumped into the well, and thenmonitoring the fluids subsequently recovered from the well to determinethe concentration of the tracer in the well fluids recovered. Theconcentration of the recovered tracer is compared with the concentrationoriginally pumped into the well, and this is used to estimate the amountof the original fracking liquid that has been recovered. Generally, oncea substantial portion of the fracturing liquid has been recovered, thewell is placed into production.

Fracturing liquids contain a number of additives and chemicals that areused to facilitate the fracturing process. Among these are specializedsand that is used as a proppant, a thickening or gelling agent thatincreases viscosity thereby enabling the water to carry the proppantinto the fractures, acid used to control pH of the well, a breakingagent that later reduces the viscosity so that the fracturing liquid canbe more readily recovered, and numerous other chemical treatment, thedetails of which are beyond the scope of this disclosure. Some considera portion of these additives and chemicals to be environmentallyquestionable, and so the movement of the fracturing liquid is monitoredwith respect to migration of the fracturing liquids into adjacentformations, possibly including fresh water resources. Thus, it is usefulto monitor migration of subterranean fluid movements by detecting thetracer in adjacent oil wells and other access points, such as nearbyinjection wells and water wells. The fracturing liquids also impedeproduction of oil and gas, and operators take a number of actions tofacilitate their removal. This may include chemical treatments to alterthe fracture liquids to enhance their removal, and also the addition offlushing liquids to dilute or alter the nature of the fracturingliquids.

Various types of tracers have been employed in hydraulic fracturingliquids. Selection and implementation of a tracer is non-trivial becauseof the cost constraints and the harsh environment that oil and gas wellspresent. The tracing material needs to be economically feasible in largescale drilling operations, it must be readily detectable at very lowconcentrations using commercially available test equipment, and it mustsurvive the extremes of pressure and temperature, and the chemical andbiological environment present in an oil and gas well. It is known touse certain chemical tracer compounds, fluorescent dye tracers,radioactive isotope tracers, fluorinated benzoic acid, ionized salts,and certain other chemicals. However, the number of discrete and uniquetracers that can be used in a single hydraulic fracturing job is quitelimited, and is generally just a handful that would be practicable in asingle fracking job. This is a significant limitation because anoperator cannot monitor a complex fracking job in detail. Many jobs useonly a single tracer, which only enables the tracing of the frackingliquids in total. Some jobs can use individual tracers for a few stagesof a fracking job. Thus it can be appreciated that there is a need inthe art for a system and method of tracing hydraulic fracturing liquidthat provides greater flexibility, greater detail, and accuracy in areliable and cost effective manner.

SUMMARY OF THE INVENTION

The need in the art is addressed by the methods of the presentinvention. The present disclosure teaches a method of tracing themovement of plural volumes of fracking liquid that are pumped into anoil or gas bearing formation through a first stage of perforations in awell casing, which is coupled to a wellhead, by using plural uniqueoligonucleotide markers. The method includes sequentially marking theplural volumes of fracking liquid with plural unique oligonucleotides,each at a predetermined concentration, and, sequentially pumping each ofthe plural volumes of fracking liquid through the first stage ofperforations and into the formation, which sequentially advances each ofthe plural volumes of fracking liquid outwardly into plural fracturezones in the formation. The method further includes pumping formationfluids to the wellhead, which may contain portions of the plural volumesof fracking liquid, while monitoring the periodic volume of the fluidspumped out. Also, periodically gathering formation fluid samples as theformation fluids are pumped to the wellhead, and, correlating each ofthe formation fluid samples to plural periodic volumes. Then, analyzingthe concentration of the plural unique oligonucleotides in each of thesamples, and, calculating the quantity of each of the plural volumes offracking liquid recovered during each of the plural periodic volumesaccording to the concentration of the plural unique oligonucleotidespresent in each of the samples. This process establishes the movement ofthe plural volumes of fracking liquids out of the corresponding fracturezones.

In a refinement to the foregoing embodiment, the sequentially markingstep is accomplished by metering liquid slurries that include the uniqueoligonucleotides into a fracking liquid blender on an ongoing basis as afracking process is occurring. In another embodiment, the sequentiallypumping each of the volumes is synchronized with plural pre-definedsub-stages of a fracking job, so that each of the sub-stages is markedwith a unique oligonucleotide. In a refinement to this embodiment, theplural sub-stages of the fracking job are distinguished from one anotherby the quantity and size of proppant incorporated into each of thevolumes of fracking liquid.

In a refinement to the foregoing embodiment, the pumping formationfluids to the wellhead step is accomplished using a down-hole pumpcoupled to the wellhead through a tubing string, and, the gatheringplural formation fluids step further includes the step of drawing fluidsfrom the tubing string at the wellhead. In a refinement to thisembodiment, where the drawing fluids from the tubing string at thewellhead step also includes automatically, and periodically, routing theplural fluid samples to plural fluid sample vessels.

In a refinement to the foregoing embodiment, the monitoring the periodicvolume of the formation fluid step further includes outputting aperiodic volumetric flow value, and, the gathering plural formationfluid samples step further includes recording a corresponding periodicvolumetric flow value for each.

In a refinement to the foregoing embodiment, the plural uniqueoligonucleotides are selected from predetermined sequences ofdeoxyribonucleic acid, ribonucleic acid, and locked ribonucleic acid. Inanother refinement, the plural unique oligonucleotides are synthesizedunique DNA sequences.

In a refinement to the foregoing embodiment, the analyzing theconcentration of the unique oligonucleotides step further includesconcentrating at least one of the plural formation fluid samples, and,measuring the atomic mass and quantity of each of the uniqueoligonucleotides therein by using a matrix-assisted laserdesorption/ionization with time of flight mass spectrometer.

The present disclosure also teaches a method of tracing fracking liquidin oil or gas bearing formations using plural unique oligonucleotidemarkers. This method includes pumping the plural volumes of frackingliquid, each marked with a unique oligonucleotide, into the formation,thereby defining plural fracture zones in the formation, and, pumpingfluids out of the formation while taking plural fluid samples. Then,analyzing the concentration of the unique oligonucleotides in each ofthe plural fluid samples, and, calculating the ratio of each of theplural volumes of fracking liquid recovered for each of the plural fluidsamples according to the concentration of the unique oligonucleotidespresent in each of the plural samples. And, then, establishing thequantity of the plural volumes of fracking liquids removed from theplural fracture zones.

In a refinement to the foregoing embodiment, the unique oligonucleotidesare biotinylated, and the analyzing the concentration step furtherincludes immobilizing avidin or streptavidin onto magnetic particles,and, mixing the magnetic particles with at least one of the plural fluidsamples, thereby enabling the formation of non-covalent bonds betweenthe biotinylated oligonucleotides and the immobilized avidin orstreptavidin. Then, removing the magnetic particles from the sample bymagnetic attraction, thereby concentrating the sample. And then,measuring the quantity of each of the unique oligonucleotides. In arefinement to this embodiment, the unique oligonucleotides arebiotinylated by binding biotin to the 5′-end of the oligonucleotides.

In another refinement to the foregoing embodiment, the measuring stepfurther includes determining the atomic mass and quantity of each of theunique oligonucleotides in the sample using a matrix-assisted laserdesorption/ionization with time of flight mass spectrometer. In anotherrefinement, the method includes agitating the at least one of the pluralfluid samples with the magnetic particles to facilitate the formation ofnon-covalent bonds. In another refinement, the recovering step isaccomplished by inserting a magnet into the sample, and then rinsing themagnetic particles prior to the measuring step.

The present disclosure teaches a method of tracing the movement ofplural volumes of fracking liquid pumped into an oil or gas bearingformation through plural stages of perforations in a well casing, whichis coupled to a wellhead, by using plural unique oligonucleotidemarkers. This method includes the steps of forming a first stage ofperforations through the casing and into the formation, and sequentiallymarking a first group of volumes of fracking liquid with a first groupof unique oligonucleotides, each at a predetermined concentration. Then,sequentially pumping each volume in the first group of volumes offracking liquid through the first stage of perforations and into theformation, thereby sequentially advancing each of the first group ofvolumes of fracking liquid outwardly into a first group of fracturezones in the formation adjacent to the first stage of perforations.Next, forming a pressure seal within the casing between the first stageof perforations and the wellhead. Then, forming a subsequent stage ofperforations through the casing and into the formation at a locationbetween the pressure seal and the wellhead, and, sequentially marking asubsequent group of volumes of fracking liquid with a subsequent groupof unique oligonucleotides, each at a predetermined concentration. Then,sequentially pumping each volume in the subsequent group of volumes offracking liquid through the subsequent stage of perforations and intothe formation, thereby sequentially advancing each of the subsequentgroup of volumes of fracking liquid outwardly into a subsequent group offracture zones in the formation adjacent to the subsequent stage ofperforations. Then, the pressure seal is removed to prepare for pumpingformation fluids to the wellhead, which may contain portions of thefirst group of volumes of fracking liquid and the subsequent group ofvolumes of fracking liquid, while monitoring the periodic volumethereof. As this occurs, periodically gathering plural formation fluidsamples as the formation fluids are pumped to the wellhead, and,correlating each of the plural formation fluid samples to pluralperiodic volumes of formation fluid. Then, analyzing the concentrationof the first group of oligonucleotides and the subsequent group ofunique oligonucleotides in each of the plural samples, and, calculatingthe quantity of each of the first group of volumes of fracking liquidand subsequent group of volumes of fracking liquid recovered during eachof the plural periodic volumes of formation fluid according to theconcentration of the plural unique oligonucleotides present in each ofthe plural samples. Thusly, establishing the movement of the first groupof volumes of fracking fluid and subsequent group of volumes of frackingliquids out of the first group of fracture zones and subsequent group offracture zones.

In a specific embodiment of the foregoing method, the sequentiallypumping each volume in the first group of volumes, and the subsequentgroup of volumes, are synchronized with plural pre-defined sub-stages ofa fracking job, such that each of the sub-stages is marked with a uniqueoligonucleotide. In a refinement to this embodiment, the pluralsub-stages of the fracking job are distinguished from one another by thequantity and size or proppant incorporated into the plural volumes offracking liquid.

In specific embodiment of the foregoing method, the monitoring theperiodic volume of the formation fluid step includes outputting aperiodic volumetric flow value, and the gathering plural formation fluidsamples step includes recording a corresponding periodic volumetric flowvalue for each.

In specific embodiment of the foregoing method, the first group ofunique oligonucleotides and the subsequent group of oligonucleotides areselected from predetermined sequences of deoxyribonucleic acid,ribonucleic acid, and lock ribonucleic acid. In another embodiment, theanalyzing the concentration of the plural unique oligonucleotides stepincludes concentrating at least one of the plural formation fluidsamples, and, measuring the atomic mass and quantity of each of theplural unique oligonucleotides therein using a matrix-assisted laserdesorption/ionization with time of flight mass spectrometer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram of the hydraulic fracturing process accordingto an illustrative embodiment of the present invention.

FIG. 2 is a system diagram of the fracking liquid removal processaccording to an illustrative embodiment of the present invention.

FIG. 3 is a system diagram of the oligonucleotide marking and pumpingprocess according to an illustrative embodiment of the presentinvention.

FIG. 4 is a system diagram of the formation fluid sampling processaccording to an illustrative embodiment of the present invention.

DESCRIPTION OF THE INVENTION

Illustrative embodiments and exemplary applications will now bedescribed with reference to the accompanying drawings to disclose theadvantageous teachings of the present invention.

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the invention is not limited thereto. Those havingordinary skill in the art and access to the teachings provided hereinwill recognize additional modifications, applications, and embodimentswithin the scope hereof and additional fields in which the presentinvention would be of significant utility.

In considering the detailed embodiments of the present invention, itwill be observed that the present invention resides primarily incombinations of steps to accomplish various methods or components toform various apparatus and systems. Accordingly, the apparatus andsystem components and method steps have been represented whereappropriate by conventional symbols in the drawings, showing only thosespecific details that are pertinent to understanding the presentinvention so as not to obscure the disclosure with details that will bereadily apparent to those of ordinary skill in the art having thebenefit of the disclosures contained herein.

In this disclosure, relational terms such as first and second, top andbottom, upper and lower, and the like may be used solely to distinguishone entity or action from another entity or action without necessarilyrequiring or implying any actual such relationship or order between suchentities or actions. The terms “comprises,” “comprising,” or any othervariation thereof, are intended to cover a non-exclusive inclusion, suchthat a process, method, article, or apparatus that comprises a list ofelements does not include only those elements but may include otherelements not expressly listed or inherent to such process, method,article, or apparatus. An element proceeded by “comprises a” does not,without more constraints, preclude the existence of additional identicalelements in the process, method, article, or apparatus that comprisesthe element.

As mentioned hereinbefore, it is important to remove as much of thefracking liquid as possible prior to placing a well into production. Thefracking liquid interferes with production for a number of reasons, oneof which is the fact that viscosity interferes with flow of reservoirfluids into the well casing. Certain chemical treatments are included inthe fracking liquid to reduce its viscosity, called breaking agents. Thebreaking agents operate over time such that the fracking liquid isviscous as it is pumped into the well, but less viscous when it ispumped out. The fracking liquid is pumped into the formation in severaldiscrete stages, which correspond to several sets of perforationsthrough the well casing, which are located at various depths within theformation. At each stage of the perforations, there are typicallyseveral sub-stages injected in the fracture process. The sub-stages mayeach have a different fracking liquid blend, most often includingdifferent proppant material configurations. For example, different sievesize sand or different amounts of sand added to each barrel of frackingliquid. As these sub-stages of fracking liquid are pumped in, they eachdefine different fracture zones within any given fracture stage. Eachsubsequent sub-stage of fracking liquid pumped into a given stage pushesthe previous stage outwardly from the casing perforations. Thus, eachzone in the fracture may have a different fracking liquid profile,generally corresponding to the sub-stages. At the time this frackingliquid is recovered from the well, the individual zones drain back intothe well casing and are pumped out. The operator of the well desires tounderstand the performance of the fracking job, including details on howindividual zones have been fractured, and how the fracking liquid fromeach has been recovered, including the volume of liquid and the timetaken for the recovery process to occur.

Wells that includes a horizontal bore into a formation commonly includeten or more perforation stages. Each stage may include from five to asmany as thirty sub-stages, which corresponds to perhaps two hundredfracture zones in a given well. Ideally, an operator would like to knowabout the removal of fracking liquid from every zone. Unfortunately,current tracer variants are far more limited in number. It would bechallenging to assemble twenty discrete tracing compounds to use in agiven well, which places a clear limit on the amount of information anoperator can garner during the fracking liquid removal process. Thereason this is challenging is because of the extreme and hostileenvironment present in an oil and gas well. In addition to presenting acomplex chemical environment, there is generally an acidic pH, highpressures, turbulent and shear forces, and high temperatures in a wellduring the fracking process. In order to function reliably, each tracercompound must survive the down-hole environment without alteration ofany kind, and each tracer should not react with any chemical compoundspresent in the well. There can also be biological and enzymatic issuesin the well that affect the tracers. In addition, the tracer compoundsmust be economically feasible, and must be detectible at very lowconcentrations (in the order of parts per billion or trillion) usingcommercially available test equipment. Furthermore, during the detectionand measurement processes, it may be necessary to remove the tracercompounds from the well formation fluid, and concentrate them, prior toperforming a test of its recovered concentration.

The present disclosure teaches the use of plural oligonucleotidecompounds as hydraulic fracture liquid tracers. The present disclosurealso presents specific handling and automation systems, as well asspecific test methodologies. These oligonucleotides includedeoxyribonucleic acid (DNA), ribonucleic acid (RNA), and locked nucleicacid (LNA), each configured with a unique sequence that can be readilydiscriminated using certain mass spectrometer test equipment andmethodologies.

Reference is now directed to FIG. 1, which is a system diagram of thehydraulic fracturing process according to an illustrative embodiment ofthe present invention. At the surface level 2, a wellhead 1 is coupledto a well casing 4, which continues downwardly to a horizontal casing 6that was drilled and installed into an oil and gas bearing geologicformation 3. In FIG. 1, the well has been drilled and cased, and fivestages 5 of perforations and fractures have been completed. The variouscomponents of the hydraulic fracturing equipment are shown on thesurface 2. The hydraulic fracturing process occurs in a coordinatedfashion, stage by stage 5, and zone by zone 7, until all of the zones 7have been fractured. Each individual zone, referenced by a combinationof its stage number 5 and its zone number 7, corresponds to a sub-stageof the fracturing process, and may also have utilized a distinctfracturing liquid mixture, and may have been marked with a uniquetracing oligonucleotide.

At the surface 2, plural hydraulic pumps 14 force fracking liquid downthe casing 4 at very high pressure. The hydraulic pumps 14 are fed mixedfracking liquid from a blender 12. The blender 12 operates on acontinuous basis during each stage 5 of the fracking job, continuallybeing fed with the various components of the particular fracking liquidmixture presently required by a fracking job specification. The frackingjob specification is generated by petroleum engineers prior tocommencement of the job, and its details are beyond the scope of thisdisclosure. With respect to this disclosure, the fracking liquid mixturecomponents are divided into water 8, chemicals 16, sand, or proppant,18, and tracer compounds 20. The water 8 is the largest portion of thefracking liquid, and it is pumped into the blender 12 by a water pump10, which supplies the water 8 at a predetermined rate according to thefracking job specification. Similarly, the sand 18 is fed on a conveyorat a predetermined rate, and enters an opening in the top of the blender12. The chemicals 16 can be fed in various manners depending on theirrespective material handling properties. The tracer compounds 20 are fedin precisely using a positive displacement metering pump 22. This isnecessary because the concentration of the tracers 20 are so small,typically on the order of parts per million, or less.

The fracking job of FIG. 1 proceeds according to a sequential schedule.In this illustrative embodiment, that fracking schedule includes fivestages 5 (labeled Stage 1 through Stage 5), each having five sub-stagesthat result in five fracture zones 7 (labeled Zone A through Zone E)each, for a total of twenty-five individual zones. Since each zone is toreceive a unique fracking liquid blend according to the frackingschedule, and since there is just the single well casing 4, 6 to serveas the fracking liquid delivery conduit, it is necessary to sequence thepreparation and delivery of the fracking liquid. Naturally, this beginswith Stage 1, which is furthest from the wellhead 1. A set ofperforations 26 are formed through the casing 6, accessing the formation3 at the location of Stage 1. The surface 2 equipment is activated, andthe fracking liquid, which also includes a unique oligonucleotide markerfor Stage 1-Zone E, is pumped down the casing 4, 6. This liquid passedthrough the perforations 26 and into the formation. On a continuouspumping basis, the subsequent four zones (Zone D, Zone C, Zone B, andZone A of Stage 1) are pumped through the perforations 26. Note thateach zone receives a distinct fracking liquid mixture according thefracking schedule, and that each also receives a unique oligonucleotidemarker. Also, note that the zones are pumped in reverse order, whereeach subsequent zone pushes the prior zone's fracking liquid outwardlyinto the formation, fracturing it as they progress. In other words, ZoneE is pumped first, followed by Zone D, Zone C, Zone B, and Zone A. WhenStage 1 is complete, a pressure seal 36 is inserted into the casing toisolate Stage 1 from the next sequence of events.

The pressure seal 36 may be a type of composite plug, as are known tothose skilled in the art. Once plug 36 is in place, then the set ofperforations 28 for Stage 2 are formed, and the next five sub-stages offracking liquid with unique oligonucleotides are pumped to form the fivefracture zones of Stage 2. Then, plug 38 is inserted to isolate Stage 2from the subsequent Stage 3. This process repeats for Stage 3, withperforations 30 and plug 40, Stage 4 with perforations 32 and plug 42,and finally Stage 5 with perforation 34. Each of the five stages 5 hasfive zones 7, and all twenty-five of the zones have a specific fractureliquid and a unique oligonucleotide disposed within fractures justformed in the formation 3.

The nature of the stages and fractures zones depends in large measure onthe nature of the formation and the petroleum engineers' plan for theextent of the fracturing job. To give this a sense of scale, someexemplary well perforation and fracturing specifics are worthconsidering. A well may be from 5000 to 20,000 feet deep with horizontalsections extending out to 7000 feet and more. Off-shore wells are evendeeper and longer. The well is drilled and then cased with steel casing,which is commonly 5.5″ in diameter. The bottom of the casing is closedin some fashion so that it holds pressure. Once the well is cased, thedrilling rig is removed, and a “wireline crew” perforates the casing atstage locations specified by the petroleum engineers. It is common touse seven to eleven stages in a single well, but other quantities areknown as well. The perforation is done with plural inverted bulletshaped copper projectiles fired with shaped charges. Each projectilemakes a 0.2 to 0.25 inch diameter hole in the casing. A single stage ofperforations is typically about twenty feet long, but shorter lengthsare used as well, and some perforations can be over one hundred feetlong.

The plugs used between stages are generally a composite material that iscompressed against the interior of the well casing to withstandpressures on the order of thousands of PSI. The plugs can later bedrilled out, however some have a dissolvable core, which opens afterseveral hours to several days later. In the case of dissolvable plugs,the fracture schedule must proceed at a pace commensurate with the rateat which the plugs dissolve.

As noted above, the fracturing process creates a false porosity in theformation. This is particularly useful in horizontal wells cut throughshale deposits. A fracture zone can extend three hundred feet from thewell casing. The sand, or proppant, holds the fractures open after thehydraulic fracking liquid pressure is removed. Various sizes of sand areutilized in the various zones. An additive is used to gel or thicken thefracking liquid because the increased viscosity enables the liquid tocarry the proppant out into the fracture zones. The number of zones ineach stage is typically in the four to ten range, but the use of as manyas thirty zones in a single stage is known. Thus in a large fracturejob, there could be fifteen stages with thirty zones each, totaling fourhundred fifty zones, each of which could be marked with a uniqueoligonucleotide.

With respect to the pumping and pressures applied during the frackingprocess, fracking liquid flow rates can run 70-75 barrels per minutewith pressures well over 7000 PSI. The pumping time for a single stagecan range from one to four hours. A typical fracking job can utilize 2million gallons of fracking liquid.

Reference is now directed to FIG. 2, which is a system diagram of thefracking liquid removal process according to an illustrative embodimentof the present invention. This figure generally corresponds to FIG. 1,after the hydraulic pressure has been removed from the well and thefracking equipment has been removed. This is the recovery phase of theproject, where the fracking liquid is removed from the formation. Thefirst step is to open the plugs of FIG. 1, which can be accomplished bydrilling or through the use of dissolvable plugs. This action may allowsome of the fracking fluids to flow out of the well due to the pressurebuilt up in the fracturing process, but generally, a down-hole pump willbe utilized to recover the fracking liquid. As the fracking liquid isremoved, it is typically mixed with formation fluids. Note that whilethe fracking liquids pumped into the well area generally free of gases,the formation fluids comprise both liquids and gases. FIG. 2 illustratesthe fracking liquid recovery process.

In FIG. 2, a down hole pump 54 has been inserted into the casing 4,which operates to pump fluids out of the formation, up the casing 6, 4,and to the wellhead 1. In this embodiment, a sucker rod 52 driven pump54 is employed, however, a submersible pump can also be used, as isknown to those skilled in the art. The sucker rod 52 couples the pump 54to a reciprocating pump jack drive unit 50 at the surface 2, as are wellknown in the art. As fluids are removed from the casing, additionalformation fluids and fracking liquids flow from the formation 3 and thefracture zones 7 into the casing 6. The wellhead 1 has a pipingarrangement that routes the liquids from a tubing string 56 and gasesfrom a casing annulus 58 to a fluid outlet 60. Samples of the fluidoutput 60 are periodically gathered for testing. This testing includestesting for the concentration of the several oligonucleotides that weremixed into the fracking liquid as the fracturing processed occurred.

It can be appreciated that the fracture liquids in the several zones 7generally flow into the casing on a last-in, first-out basis, and thetesting of oligonucleotides may demonstrate this general trend. However,that assumption would only hold true for a uniform formation withconsistent porosity and uniform formation pressures. Further, suchuniform flow would require that the consistency and break-down of thefracking liquid viscosity was uniform throughout the several zones. Inreality, these assumptions would be very unlikely to hold true. Thereare many variables that affect the nature and rate at which the fractureliquids are recovered. First is the material and consistency of theformation, and the extent of hydrocarbon and brine fluids therein. Thesetwo factors are of interest to the operator, because they are indicatorsof the production potential of the well and also indicate the generalnature of the reserve, which influences how nearby wells might beengineered. Another factor is the content of the fracture fluid mixturein each of the several stages. There can also be problems in therecovery process where certain stages do not readily release thefracking liquid, and therefore limit production potential for the well.The oligonucleotide concentration can indicate such problematic areas,and suggest alternative treatments for mitigating them.

Ideally, the well operator's goal is to remove all of the frackingliquid from the well, so that the well only produces formation fluids.In an exemplary well, approximately 2 million gallons of fracking liquidare used, and the recovery process goal is to remove all of this so thatthe well can be placed into production of oil and/or gas. In a typicalwell, perhaps 75% of the fracking liquid is actually recovered. It isuseful to understand which of the plural zones' fracking liquid has beenrecovered, and where the 25% of unrecovered fracking liquid might be.This is only possible if all of the fracking liquid zones have beenuniquely and discretely marked. With respect to when the well istransitioned from recovery of fracking liquids to production of oil andgas, once the toe perforation start to flow back, then it can be assumedthat the well is ready for production. This is because the toeperforation was the last to be fractured, and will be the last toproduce. Therefore, once this perforation starts to produce, then thewhole well is likely to be ready for production. The uniqueoligonucleotides that marked the toe perforation stages will indicate tothe operator when that stage is beginning to flow.

In an exemplary embodiment, well fluid samples are taken on a periodicbasis, which gradually lengthens over time. For example, during thefirst day of recovery, a first sample can be taken shortly after therecovery pump starts operating, and then samples may be taken atfour-hour intervals. The second day samples may be taken at eight-hourintervals, then twelve-hour intervals the next day, until just dailysamples are taken. This can go on for a month, or until testing showsthat most of the fracking liquids have been recovered. The rate at whichfracking liquid and formation fluids are pumped out of the well varieswidely, based on the characteristics of the formation. This may rangefrom 1 bbl/day to 2000 bbl/day. In the exemplary well, the recovery rateis approximately 300 bbl/day. At initial pumping, the recovered fluidsare nearly all fracking liquid, but by the end of the recovery period,only a small fraction of the pumped formation fluids is fracking liquid.Again, the oligonucleotide testing procedure provides detailedinformation on the rate of fracking liquid recovery.

Reference is now directed to FIG. 3, which is a system diagram of theoligonucleotide marking and pumping process according to an illustrativeembodiment of the present invention. This figure illustrates theequipment at ground level 62 used to pump the fracking liquid into thewellhead 64 and down the casing 65. The water flows from an input pump76, which is supplied from a high volume reservoir (not shown), and intoa blender 74. The blender 74 has mechanical agitators inside, whichcombine and mix the water with sand and chemicals (not shown) on acontinuous basis. In the illustrative embodiment the blender 74 has amixing volume of approximately one hundred barrels. The volume offracking liquid flowing out of the blender 74 is measured by a flowmeter 72, which is used to monitor and maintain the volumetric flowsaccording to the fracking schedule, and for general record keepingrequirements. An input manifold 70 routes the fracking liquid to pluralhigh-pressure fracking pumps 68. The outlets of the plural high-pressurepumps 68 are combined by an outlet manifold 66, which is coupled to thewellhead 64.

As was noted hereinbefore, petroleum engineers develop a frackingschedule that itemizes the mixture components of the several zones ofeach stage of a fracking job. This schedule is used as the basis foradding oligonucleotides into the blending process in concert with theother blended components. The individual zones are each marked with aunique oligonucleotide. Therefore, in FIG. 3, there are plural tracertanks 82 that each contains a unique oligonucleotide. Each of the pluraltracer tanks 82 is coupled to a corresponding metering pump 84. Themetering pumps 84 run at fairly low volumetric rates, so peristalticpumps are a suitable choice for this application. The output of theplural metering pumps 84 are combined by a manifold 86 and coupled tothe blender 74 or the water feed line 88 into the blender 74.

Because the fracturing process is implemented on a continuous basis, andbecause there is a predetermined fracking schedule, the pumping of theoligonucleotides 82 can be automated. In the illustrative embodiment,the stage schedule 80 contains a database of the volumetric flow foreach zone of every stage, and also the type and concentration for eachof the discrete oligonucleotides. A controller 78, such as an industrialprogrammable logic controller, monitors the flow meter 72 and the stageschedule 80, and then activates the appropriate metering pump 84 so thatthe correct amount of oligonucleotide is pumped to yield the specifiedinput concentration, which may be approximate one to five parts permillion in the illustrative embodiment. Note that oligonucleotide isproduced as a fine dry power. To facilitate the metering and pumpingoperations, the oligonucleotides are mixed with fresh water into highconcentration slurry, and are then placed into the tracer tanks 82.Agitation may be required to maintain a uniform slurry concentration inthe tracer tanks 82.

Reference is now directed to FIG. 4, which is a system diagram of theformation fluid sampling process according to an illustrative embodimentof the present invention. This figure illustrates a more detailed viewof the well fluid sampling system, and also shows an automated samplingembodiment. At the ground level 90, the wellhead comprises the wellcasing 92, a tubing string 94, and the sucker rod 96, which drives thedown-hole pump. Generally, fluids are pumped up the tubing string 94,and gases flow up the casing 92 annulus. Although, the well fluids oftentimes have a high percentage of gas content, as is know to those skilledin the art. A fluid pipeline 98 is coupled to the tubing string 94, anda gas pipeline 100 is coupled to the casing 92 annulus. Suitable valvesare used, and the well fluids are output 102 to a storage ortransportation system (not shown). The illustrative embodiment utilizesa sampling line 104 connected to the fluid pipeline 98, which is used todraw periodic samples of the well fluids, which would include some ofthe fracking liquids.

In the automated sampling embodiment of FIG. 4, the sampling isaccomplished periodically and automatically using a solenoid valve 106under control of an industrial programmable controller 110. Atpredetermined intervals, the controller 110 opens the solenoid valve 106to allow well fluids to pass into the valve body 108. The valve body 108automatically routes each sample of well fluid to a predetermined samplevessel 112. An operator periodically visits the well site to retrievethe sample vessels 112, and replace them with empty vessels. Thisarrangement facilitates more accurate sample gathering and less operatorinvolvement. Once the samples are gathered, they are ready forprocessing and measurement of the concentrations of the pluraloligonucleotides originally pumped in with the fracking liquid.

Once the samples are gathered from the wellhead, testing for theconcentrations of the plural oligonucleotides is undertaken, and thencalculations are made to establish the volume of fracking liquids thathave been removed per sample period. These values, gathered over theseveral sampling periods, are then used to establish the totality of thefracking liquid recovery process, which is presented in table form forthe well operator's uses. It will be appreciated by those skilled in theart that the raw well fluids are challenging to deal with, and are hardon all the instruments that are used in the sampling and measuringprocess. These fluids contain brine, crude oil, dissolved gases, gasbubbles, acids, solids, various well chemicals, the fracking liquid, andthe oligonucleotide tracers. The raw well fluids are not ready fortesting in a spectrometer, as least not on an ongoing, commercial basis.

In the illustrative embodiment, oligonucleotides are added to thefracking liquid to serve as the tracer material. In order to gatheruseful information in the testing process, the testing equipment needsto accurately measure minute concentrations of these materials.Additionally, these materials must survive the harsh down-holeenvironment. Tests conducted in developing this disclosure indicatedthat oligonucleotides do endure the down-hole environment and are usefulfor tracing fracking liquid. Oligonucleotides are short, single-strandedDNA or RNA molecules. They are typically manufactured in the laboratoryby solid-phase chemical synthesis. These small bits of nucleic acids canbe manufactured with any user-specified sequence. The number ofpotential sequences is very large. The number of sequences is four tothe power of N, where N is the length of the sequence. The length of thesequence can range from 2 to 150, which equates to tens of thousands ofdiscrete and unique oligonucleotide sequences. Each sequence has adiscrete atomic mass, which is what is measured to identify uniquesequences. The range of molecular weights for these oligonucleotides isfrom 3000 to 6500 atomic mass units.

As was noted hereinbefore, the oligonucleotides contemplated in theillustrative embodiment are DNA, RNA, and LNA. LNA is an acronym forlocked nucleic acid. LNA is also referred to as inaccessible RNA, and isa modified RNA nucleotide. During synthesis, the ribose moiety of an LNAnucleotide is modified with an extra bridge connecting the 2′ oxygen and4′ carbon. The bridge “locks” the ribose. The locked ribose conformationenhances base stacking and backbone pre-organization. This significantlyincreases the melting temperature of oligonucleotides, making them moretolerant in the down-hole environment. With respect to down-holedurability of these oligonucleotides, testing indicates that LNA is mostdurable, then RNA, and then DNA. However, DNA can be utilized down-holeand show good durability. Tests establish that DNA is thermally stableto 1000 degrees, and will not shear under wellbore pressures to at least7700 PSI. It is expected that DNA can out-survive casing static pressurelimits of 20,000 psi. The highest risk to the integrity of the DNAmolecules are enzymes called DNAase. However, test samples showed thatonly the DNA samples sent down hole were detected in well fluid, with nobyproducts from DNAase. Furthermore, testing with certain massspectrometer test methodologies showed that DNA could be reliablydetected after exposure to the down-hole environment. DNA is highlytolerant to temperatures seen down-hole, and also tolerant to a widerange of pH. While very low pH for extended periods of time can damageDNA, the down-hole environment is usually not that acidic. The down-holepH may be in the 5-6 range, with pH of 4 being a practical low limit foracidity. However, DNA can tolerate a pH of 3 for reasonable periods oftime. It would take long-term exposure to damage oligonucleotides atsuch pH levels.

Having established that oligonucleotides are suitable for tracingfracking liquids in real-world down-hole environments and time frames,the next hurdle to their application is recovery and testing for minuteconcentrations present in well fluids. Since the oligonucleotides wouldbe destroyed by flame (gas chromatograph), the testing procedure mustuse a non-flame type of mass spectrometer. In the illustrativeembodiment, a matrix-assisted laser desorption/ionization source with atime-of-flight mass analyzer (MALDI-TOF) mass spectrometer is utilized.This instrument tests a dry sample, so it is necessary to reduce andconcentrate the well fluid sample in order to conduct the measurementsof oligonucleotide concentrations. A MALDI-TOF mass spectrometer isaccurate to +/−0.2%, and can readily distinguish the oligonucleotidesequences discussed herein. The output of MALDI-TOF is spectrographstyle graphic, where the horizontal line distinguishes individualoligonucleotide masses and the vertical amplitude indicates the totalmass of each oligonucleotide in a given test run. This data can, orcourse, be quantified for analysis and incorporation in the test resultsfor the well operator.

The challenge of isolating the oligonucleotides from the other wellfluid materials is addressed by biotinylation. This simplifies therecovery of the oligonucleotide in the well fluid samples and increasesthe overall sensitivity of the testing processes. This is accomplishedby biotinylating the 5′-end of the sequence of the oligonucleotidesbefore they are added to the fracking liquid and pumped down-hole.Biotinylation takes advantage of the fact that biotin and avidin orstreptavidin (hereafter collectively referred to as “avidin”) form thestrongest non-covalent bond known in nature with a dissociation constantof greater than ten to the minus fifteenth power. Once the well fluidsamples are collected, they are infused with magnetic particles thathave avidin immobilized onto their surfaces. Of course the biotinylatedoligonucleotides and avidin coated magnetic particles are stronglyattracted to one another. This attraction is facilitated by agitatingthe mixture for a period of time to insure that substantially all of thebiotin and avidin have bonded, and therefore assuring that all of theoligonuceotides have been attached to the magnetic particles.

After agitating the sample for a given period to ensure that thebiotinylated oligonucleotide has had sufficient opportunity tophysically contact the avidin (or streptavidin) magnetic particles, apolar magnet is inserted into the sample, which easily gathers all ofthe magnetic particles that have the oligonucleotides bonded to them.The magnetic particles are washed to removed well fluid residue, andfurther washed to collect the magnetic particles from the magnet. Themagnetic particles are collected in a small volume allowing forsubsequent washing with deionized water to remove any residualcomponents from the sample solution. The magnetic particles are thenready for further preparation for analysis by, preferably, adelayed-extraction (DE) matrix-assisted laser desorption/ionization(MALDI) time-of-flight (TOF) mass spectrometer.

With respect to suitable sample sizes and test concentrations, tracersare added to the fracking liquid with a concentration in the range ofone to five parts per million. The sample taken from the well fluid flowmay be in the range from four ounces to one gallon, which isconcentrated, dried, and then measured with a DE-MALDI-TOF massspectrometer. Sample concentrations of eight parts per billion arereliably detected, and concentrations below one part per billion can bedetected through the foregoing process. Further, the MALDI-TOF massspectrometer can measure thresholds as low as one part per trillion.

Thus, the present invention has been described herein with reference toa particular embodiment for a particular application. Those havingordinary skill in the art and access to the present teachings willrecognize additional modifications, applications and embodiments withinthe scope thereof.

It is therefore intended by the appended claims to cover any and allsuch applications, modifications and embodiments within the scope of thepresent invention.

What is claimed is:
 1. A method of tracing liquid in oil or gas bearingformations using unique oligonucleotide markers, comprising the stepsof: pumping a volume of liquid, marked with a unique oligonucleotide,into the formation, thereby defining a zone in the formation that hasbeen marked with the unique oligonucleotide; pumping fluids out of theformation while taking plural fluid samples; analyzing the concentrationof the unique oligonucleotide in each of the plural fluid samples;calculating the ratio of the volume of liquid marked with the uniqueoligonucleotide recovered for each of the plural fluid samples accordingto the concentration of the unique oligonucleotide present in each ofthe plural samples, and thereby establishing the quantity of the volumeof liquid marked with the unique oligonucleotide removed from the zonefor each of the plural samples.
 2. The method of claim 1, and whereinthe unique oligonucleotide marker is biotinylated, and wherein saidanalyzing the concentration step further comprises: immobilizing avidinor streptavidin onto magnetic particles; mixing the magnetic particleswith at least one of the plural fluid samples, thereby enabling theformation of non-covalent bonds between the biotinylatedoligonucleotides and the immobilized avidin or streptavidin; removingthe magnetic particles from the at least one of the plural fluid samplesby magnetic attraction, thereby concentrating the sample, and measuringthe quantity of the unique oligonucleotides.
 3. The method of claim 2,and wherein: the unique oligonucleotides are biotinylated by bindingbiotin to the 5′-end of the oligonucleotides.
 4. The method of claim 2,and wherein said measuring step further comprises the steps of:determining the atomic mass and quantity of the unique oligonucleotidesin the sample using a matrix-assisted laser desorption/ionization withtime of flight mass spectrometer.
 5. The method of claim 2, furthercomprising the step of: agitating the at least one of the plural fluidsamples and the magnetic particles to facilitate the formation ofnon-covalent bonds.
 6. The method of claim 2, and wherein the removingstep is accomplished by inserting a magnet into the sample, and furthercomprising the step of: rinsing the magnetic particles prior to saidmeasuring step.